In higher plants, nitrogen assimilated from the soil must be incorporated into organic form for transport to the growing plant. The amides, asparagine and glutamine, are the two major nitrogen transport compounds in most higher plants (Sieciechowicz et al., 1988, Phytochemistry 27:663-671). These amides function to deliver nitrogen to and from plant organs at various stages of plant development. Since the production of asparagine and glutamine for nitrogen transport is crucial to plant growth, the enzymes involved in their synthesis are targets for herbicide action. Much is known about plant glutamine synthetase (GS) as an enzyme, a herbicide-target and as a gene family. In contrast, very little is known about plant asparagine synthetase (AS) at the biochemical level, at the herbicide-target level and nothing is known about the AS gene(s) in higher plants.
Asparagine is the major nitrogen transport compound which is synthesized when a plant is faced with excess ammonia rather than nitrate. During normal plant growth, conditions of ammonia excess arise when (1) plants are treated with externally applied fertilizers; (2) plants develop nitrogen-fixing root nodules; (3) the seed storage proteins of cotyledons are deaminated during germination; and (4) senescing plants are mobilizing nitrogen for seed formation. In certain species asparagine can account for up to 86% of transported nitrogen in the above contexts. See, Lea & Miflin, 1980, Transport And Metabolism Of Asparagine And Other Nitrogen Compounds Within The Plant, in The Biochemistry Of Plants, Volume 5, Ed. Miflin, Acad. Press, New York, Ch. 16.
There are several reasons why asparagine is preferred as a nitrogen transport/storage compound compared to glutamine in situations requiring the assimilation and transport of large amounts of nitrogen. Asparagine contains a high N:C ratio (2N:4C) compared to glutamine (2N:5C) which makes asparagine a more economical nitrogen transport/storage compound compared to glutamine. Asparagine is also a preferred compound for nitrogen transport/storage because it is relatively inert compared to glutamine. Because glutamine is such an active metabolite which donates the amide nitrogen to a large number of substrates, over-production of glutamine could seriously upset plant metabolism. Storage of high levels of nitrogen containing compounds is extremely important in vegetative structures such as fruits where asparagine stored in vacuoles serves as the primary source of nitrogen for seed storage protein synthesis in developing seeds.
Despite the crucial role of asparagine synthetase during plant development, very little is known about the AS enzyme in higher plants. Years of labor intensive biochemical investigations on plant AS have failed to produce a homogeneous enzyme preparation. The inability to purify AS biochemically is due, in part, to the fact that AS is extremely unstable in vitro in these partially purified extracts. In addition, AS activity is difficult to detect in partially purified extracts due to contaminating asparaginase activity, and due to the presence of specific non-protein inhibitors of AS activity.